After you kill the second king, the Master Clockwork Protocol comes to life and tries to kill you, however he is stuck to the ceiling and cannot come down - you can range attack him or stand on the metal piece jutting out of the floor to melee him. Battle him down to 10% then use the energized gear you got from 2nd king to destroy the Master Clockwork Protocol's stoneskin. Now you can finish him off to get your update for the MCP power cell (non-item).

Head back to Princess Saphronia in Kelethin and see if the power cell works on Sir Gearheart.